The purpose of this proposal is to examine the role of vitamin d in the regulation of Ca transport across the renal tubule of the rabbit. The specific questions to be investigated are: (1) Does vitamin D-deficiency decrease active Ca transport across the isolated perfused rabbit connecting tubule? (2) Does parathyroid hormone increase active Ca absorption actross isolated perfused connecting tubules from vitamin D-deficient rabbits? (3) Does vitamin D-deficiency increase the renal clearnance of Ca in the rabbit? Although 1,25-dihydroxyvitamin D3 has been shown to promote intesttinal Ca absorption, its role in renal tubular calcium transport is unclear. The recent localization of vitamin D-de-pendent Ca-binding protein in the rabbit connecting tubule suggests that this portion of the nephron may be an important site of action of vitamin d in regulating renal tubular Ca transport. connecting tubules will be dissected from the kidneys of normal, vitamin D-deficient, and vitamin D-replete rabbits and perfused in vitro with physiologic salt solutions. The ability of the epitelium to generate and maintain a Ca concentration gradient will be determined by measuring the collected tubule fluid and bathing solution Ca ion activities and total chemical Ca concentrations when tubules are perfuesed slowly. Ca ion activities and total chemical Ca concentrations when tubules are perfused slowly. The maximal rate of Ca transport will be assessed by measuring transepithelial bidirectional fluxexs with 45Ca, while the tubules are perfuesed at a rapid rate. Simultaneously, transepithelial voltage will be measured. The effect of parathyroid hormone on Ca transport will be tested by adding it to the bath surrounding the perfused tubules. Cabinding protein will be measured in individual rabit connecting tubule segments by radioimmunoassay. Renal Ca handling by the intact rabbit will be assessed by metabolic balance studies. The renal handling of Ca contributes to Ca homeostasis. the connecting tubule is an important site of Ca absorption. This study should elucidate the role of vitamin D in the regulation of Ca transport across the connecting tubule and the role of vitamin D in the renal handling of calcium.